A public concern pertaining to patient safety is that hospitals may currently be providing inadequate nurse staffing, consequently jeopardizing the quality of patient care. Recently, several studies have examined the relationship between nurse staffing and patient outcomes. Whereas these studies have produced empirical findings supporting this relationship, this study examines the cost-benefits of increasing nurse staffing as well as the nurse staffing-patient outcomes relationship. The specific aims of this study are: (1) to examine hospital characteristics that affect nurse staffing; (2) to examine the effects of nurse staffing on the occurrence of adverse events; (3) to assess the consequences of adverse events on morbidity, mortality and medical costs and decreased medical care costs due to decreases in adverse events as the benefits of increasing nurse staffing. A cross-sectional study design will be conducted using two existing databases of California acute care hospitals. Nurse staffing will be measured by six indicators, including nursing intensity-adjusted nursing hours per patient worked by registered nurses, all nursing personnel, and agency or contract personnel, skill mix, overtime index, and labor productivity. Adverse events include adverse drug events, patient falls, pressure ulcers, and nosocomial infections. These adverse events will be detected by using the International Classification of Diseases, 9th Revision diagnosis codes. Morbidity attributed to adverse events will be indirectly measured by prolonged length of stay. Analyses will employ multiple regression, logistic regression, or mixed models, depending on the dependent variables. Patient characteristics will be controlled for to isolate the effects of nurse staffing on patient outcomes. The cost benefit analysis from the hospital perspective will examine whether the benefits of increasing nurse staffing are greater than its costs. This study will provide guidelines for determining appropriate nurse staffing levels in hospitals with differing characteristics, such as patient populations and types of care provided.